If a powder begins at rest, then a stochastic model for the packed
grains is required to represent the unpredictability of the physical process
which produced the initial packing. In the best circumstances, a large
number of specimens can be prepared by that physical process and
imaged by X-ray [1]}, [2]}, [3]}, [4]}, NMR [5]},
or confocal [6]}, [7]} methods. From these images,
initial states can be selected at random directly from the physical
ensemble for use in stochastic models of flow.
